1. Field of the Invention
The present invention relates to a magnetoresistive (MR) element that stores data of “1” or “0” using the magnetoresistive effect and a magnetic random access memory (MRAM) in which a memory cell is configured by applying such a function of the MR element and, more particularly, to a spin-transfer multilevel MRAM and a write method of the same.
2. Description of the Related Art
Research for putting a magnetic random access memory using the tunneling magnetoresistive (TMR) effect into practical use is being extensively made all over the world. Freescale Semiconductor of the U.S. has applied the technique for mass production and sells MRAM chips so far although the scale is as small as 4 Mbits (see, e.g., ISSCC 2000 Technical Digest, p. 128, “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”).
According to the Julliere's model, the TMR effect of a magnetic tunnel junction (MTJ) is explained as follows: Based on the assumption that the direction of electron spins remain unchanged during the tunneling process, when magnetizations of two ferromagnetic electrodes, which sandwich a nonmagnetic layer, are directed parallel, majority-spin electrons tunnel into the majority-spin band of the other electrode, or minority-spin electrons tunnel into the minority-spin band of the other electrode.
On the contrary, when magnetizations of the ferromagnetic electrodes are directed antiparallel, the majority-spin (or minority-spin) electrons tunnel into the minority-spin (or majority-spin) band of the other electrode.
This makes a tunnel resistance (Rp) with the parallel magnetization different from a tunnel resistance (Rap) with the antiparallel magnetization. The ratio of change (the magnetoresistive effect ratio, or MR ratio) is represented byMR ratio=(Rap−Rp)/Rp=2P1P2/(1−P1P2)Pα=(Dα↑(Ef))−Dα↓(Ef))/(Dα↑(Ef))+Dα↓(Ef)), α=1,2where P is a spin polarization ratio. P is defined as the state density D↑(Ef) of the majority-spin band and the state density D↓(Ef) of the minority-spin band at the Fermi level Ef of the electrode.
In 1995, an MTJ element with alumina used as a tunnel barrier layer and a polycrystalline transition metal ferromagnetic material as an electrode layer was manufactured in Tohoku University. This MTJ element attracted a great deal of attention because it achieved an MR ration of 18% at room temperature, which was a very high MR ratio at that time.
In an MRAM, recorded data is read by detecting the change in resistance of an MTJ element caused by the TMR effect. A memory cell is designed such that the magnetization direction in one ferromagnetic layer of the MTJ element is fixed along one direction (this layer is also called a fixed layer or pinned layer), and the other magnetic layer (also called a recording layer or free layer) is given uniaxial anisotropy in the same direction as the fixed layer, and its magnetization can be reversed to be from/to parallel and to/from antiparallel to the fixed layer by a relatively weak external magnetic field. Hence, selective writing to the memory cell is possible.
When configuring a memory cell array, bit lines and word lines are arranged to be perpendicular to each other, and the MTJ element serving as a memory cell is placed at each intersection of these lines. A current is supplied to a bit line and word line corresponding to a selected memory cell, thereby generating a current magnetic field. Consequently, data can be written in only the memory cell at the intersection of the selected word line and selected bit line.
In general, the MTJ element is given shape magnetic anisotropy by making it a rectangle or ellipse in its planar shape, and is also given a resistance to the thermal disturbance by defining the magnetization direction of the element. The product of the sum of the shape magnetic anisotropic energy and induced magnetic anisotropic energy of the MTJ element and the volume of the free layer of the MTJ element is Ku×V. The induced magnetic anisotropy of the free layer of the MTJ element is aligned with the shape anisotropy, thereby preventing dispersion of the anisotropies.
The magnitude, however, of the induced magnetic anisotropy of NiFe for the free layer (a few Oe) is generally smaller than that of the shape anisotropic magnetic field (a few tens of Oe) by an order of magnitude, therefore the magnetic shape anisotropy presumably plays the most part in determining almost determines the thermal disturbance resistance and a switching magnetic field.
The switching magnetic field Hsw required to rewrite information held through magnetization in the free layer is generally given byHsw=4π×Ms×t/F (Oe)  (2)where Ms is the saturation magnetization of the free layer, t is the thickness of the free layer, and F is the width of the free layer. The sum Ku of the magnetic shape anisotropic energy and induced magnetic anisotropic energy is generally given byKu=Hsw×Ms/2  (3)
As can be seen, the width F of the free layer must be decreased in order to reduce the cell size of the conventional magnetic-field-writing MRAM cell.
Unfortunately, there is a lower limit of the thickness t of the free layer regarding the reliability issues, resulting in large Hsw and an accordingly large write current, which imposes a limit on miniaturization.
An MRAM using the operating principle of spin-transfer magnetization switching is expected as a technique that solves such a problem and realizes a larger scale memory. For such a spin-transfer-magnetization-switching MRAM, electrons having the same spin direction as the fixed layer are conducted from the fixed layer to the free layer having spins of the opposite direction in order to switch the antiparallel state to the parallel state. When the current density exceeds JcP→AP, magnetization reversal of the whole free layer is occurred, resulting in the parallel state.
By contrast, in order to switch the parallel state to the antiparallel state, electrons having the same direction as the fixed layer are conducted from the free layer to the fixed layer. As a result, electrons which have been reflected and have spins of the opposite direction to the spin direction of electrons in the free layer enters the free layer. When the current density exceeds JcAP→P, magnetization reversal of the whole free layer is occurred, resulting in the antiparallel state.
As the reading operation is the same with a conventional MRAM cell having the TMR structure in which the magnetic field is used for writing, memory cell data can be read out by reading the change of the cell resistance.
When using the spin-transfer TMR structure for an MRAM cell, current densities JcP→AP and JcAP→P required for magnetization switching are determined by the type, anisotropy, and thickness of materials for the fixed layer and free layer, etc. Therefore, the smaller the elements are, the smaller the total current required for writing and it can be said that this nature is suitable for device scaling.
Such a memory cell for the spin-transfer MRAM requires only two terminals for the reading and writing. This simple requirement can realize simplified memory cell configuration and increased density of memory cells because no write word line is necessary.
In a recently reported spin-transfer MRAM, a current is conducted perpendicularly to the TMR-effect-exhibiting films (the TMR structure) to inject spins of selected direction into the free layer in order to reverse the magnetization direction. When used for a spin transfer MRAM cell, a perpendicular magnetization film only needs to have uniaxial anisotropy perpendicular to the film plain surface (film surface), and has no needs to have shape magnetic anisotropy along the in-plan direction.
In principle, therefore, the MTJ element can have aspect ratio of 1, and can be small as possible as defined by the processing limit. Also, this MTJ element does not need the interconnects for generating the biaxial current magnetic fields along the direction parallel to the film surface (in-plane direction), and can be operated as long as the two terminals connected to the upper and lower electrodes of the TMR structure are provided. Accordingly, the cell area per bit can be reduced (see, e.g., W. C. Jeong et al., “High scalable MRAM using field assisted current induced switching”, 2005 VLSI Sympo. Technical Digest, pp. 184-185).
The switching current for the spin transfer for the in-plane-magnetization film in the above-mentioned TMR structure is given by
                              I          C                      P            -            AP                          ≈                                            A              ⁢                                                          ⁢              α              ⁢                                                          ⁢                              M                S                            ⁢              V                                                      g                ⁡                                  (                  0                  )                                            ⁢              p                                ⁢                      (                          H              +                              H                dip                            +                              H                                  k                  //                                            +                              2                ⁢                π                ⁢                                                                  ⁢                                  M                  S                                                      )                                              (        4        )                                          I          C                      AP            -            P                          ≈                                            A              ⁢                                                          ⁢              α              ⁢                                                          ⁢                              M                S                            ⁢              V                                                      g                ⁡                                  (                  π                  )                                            ⁢              p                                ⁢                      (                          H              +                              H                dip                            -                              H                                  k                  //                                            -                              2                ⁢                π                ⁢                                                                  ⁢                                  M                  S                                                      )                                              (        5        )            
Where Ms is the saturation magnetization of the free layer, V is the volume of the free layer, α is the Gilbert damping constant of the free layer, A is a constant relating to a transport model, H is an magnetic field (in the in-plane direction) applied to the wafer, Hdip is a leakage magnetic field (in the in-plane direction) from the fixed layer, P is the spin polarization ratio, Hk// is the anisotropic magnetic field (in the in-plane direction), and g is a coefficient relating to the relative angle between the free and fixed layers.
On the other hand, the switching current for the spin transfer for the perpendicular-magnetization film in the TMR structure is given by
                              I          C                      P            -            AP                          ≈                                            A              ⁢                                                          ⁢              α              ⁢                                                          ⁢                              M                S                            ⁢              V                                                      g                ⁡                                  (                  0                  )                                            ⁢              p                                ⁢                      (                                          H                                  k                  ⊥                                            -                              4                ⁢                π                ⁢                                                                  ⁢                                  M                  S                                            -              H              -                              H                dip                                      )                                              (        6        )                                          I          C                      AP            -            P                          ≈                                            A              ⁢                                                          ⁢              α              ⁢                                                          ⁢                              M                S                            ⁢              V                                                      g                ⁡                                  (                  π                  )                                            ⁢              p                                ⁢                      (                                          -                                  H                                      k                    ⊥                                                              +                              4                ⁢                π                ⁢                                                                  ⁢                                  M                  S                                            -              H              -                              H                dip                                      )                                              (        7        )            
Where Ms is the saturation magnetization of the free layer, V is the volume of the free layer, α is the Gilbert damping constant of the free layer, A is a constant relating to a transport model, H is an magnetic field (in the perpendicular direction) applied to the wafer, Hdip is a leakage magnetic field (in the perpendicular direction) from the fixed layer, P is the spin polarization ratio, Hk⊥ is the anisotropic magnetic field (in the perpendicular direction), and g is a coefficient relating to the relative angle between the free and fixed layers (see, e.g., S. Mangin et al., Nature Materials, Vol. 5, Mar 2006).
As can be seen, the spin switching current Ic is an important parameter in the spin-transfer MRAM.
Recently, it has been theoretically and experimentally verified that polycrystalline MgO with (001) crystal surface is sandwiched by polycrystalline CoFeB also with (001) crystal surface to form CoFeB(001)/MgO(001)/CoFeB(001) structure for a TMR tunnel barrier, and such a structure used as a TMR tunnel barrier can function as a spin filter which allows only Δl electrons (or, s-electrons)to penetrate it, which is called the coherent tunneling. As a result, this material is found to be a material system that can greatly contribute not only to achievement of a high TMR but also to improvement of spin injection efficiency, and therefore is expected to be put into practical use as an unnecessary material for the spin-transfer MRAM (see S. Yuasa et al., Appl. Phys. Lett. 87, 222508 (2005), K. Tsunekawa et al., “Giant Magnetoresistance Tunneling effect in low-resistance CoFeB/MgO(001)/CoFeB Magnetic Tunnel Junctions for read-head applications”, Appl. Phys. Lett. 87, 072503 (2005), H. Kubota et al., “Evaluation of Spin-Transfer Switching in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions”, Jpn. J. Appl. Phys. 44, pp. L1237-L1240 (2005).
The spin-transfer magnetization switching, however, realizes reduced memory cell size indeed, but one cell can record only one bit. Therefore, the scale of the memory can be increased only by reducing the cell size. Since the cell size reduction is restricted by the processing limit, the memory scale cannot greatly improve. Accordingly, demands have arisen for a magnetic random access memory that needs only two terminals for a memory cell for writing and reading and that realizes recording of multilevel data in one cell so as to increase the integration per unit area, and for a writing method of such a memory.